The present invention relates to a wide-angle, fast zoom lens system which is compact and has high optical performance.
A great variety of zoom lens systems which enable photography in the range of from the wide-angle region to the semi-telephoto region have heretofore been proposed. In recent years, zoom lenses raised in the aperture ratio have also actively been developed. These conventional fast zoom lens systems are of the type in which a negative lens unit is used as a front lens unit, as represented by Japanese Patent Application Laid-Open (KOKAI) Nos. 63-241511 (1988) and 02-158708 (1990). Zoom lens systems having a raised aperture ratio and an extended wide-angle region include those disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 61-275809 (1986).
Most of the fast, wide-angle zoom lens systems which have heretofore been proposed are of the type in which the 1-st lens unit has a negative refractive power and characterized in that the 1-st lens unit performs non-linear movement during zooming. This type of zoom lens system generally has an advantageous in that the diameter of the front lens can be reduced. It suffers, however, from the disadvantage that the zoom ratio cannot be raised to a very high level even if a rear lens unit is divided into sub-lens units so that a multiplicity of lens units are moved to effect zooming. The conventional zoom lens systems are also disadvantageous in that as the aperture ratio becomes higher, it becomes difficult to make effective aberration correction in the telephoto region. The reason for this is as follows: Since the 1-st lens unit has a negative refractive power, the bundle of rays incident on the 2-nd lens unit is divergent. Therefore, the aperture ratio considerably increases at the 2-nd lens unit, so that it becomes difficult to effect aberration correction satisfactorily. In addition, it has heretofore been a common practice to effect focusing by moving the 1-st lens unit, and therefore the conventional zoom lens systems have a disadvantage in that aberration variation becomes remarkable during focusing from infinity to a finite object point in the telephoto region. On the other hand, the conventional zoom lens systems use aspherical surfaces for attaining the required optical performance and have a large overall size.
To solve the above-described problems of the conventional wide-angle, fast zoom lens systems, the present invention provides a wide-angle, fast zoom lens system which is compact and has high optical performance by conducting:
(1) reexamination of the types of zoom lens; PA1 (2) examination of the refractive power distribution; PA1 (3) reexamination of the thick lens arrangement; and PA1 (4) examination of the focusing method. PA1 f.sub.1 : the focal length of the 1-st lens unit; PA1 f.sub.12W : the composite focal length of the 1-st and 2-nd lens units at the wide end; PA1 f.sub.3 : the focal length of the 3-rd lens unit; PA1 f.sub.4 : the focal length of the 4-th lens unit; PA1 .gamma.: the zoom ratio (=the focal length at the tele end divided by the focal length at the wide end) PA1 X.sub.2 : the amount of movement of the 2-nd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; PA1 X.sub.3 : the amount of movement of the 3-rd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; and PA1 X.sub.4 : the amount of movement of the 4-th lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end. PA1 .beta..sub.2T : the lateral magnification of the 2-nd lens unit at the tele end. PA1 .beta..sub.5 : the lateral magnification of the 5-th lens unit. PA1 f.sub.1 : the focal length of the 1-st lens unit; and PA1 f.sub.3 : the focal length of the 3-rd lens unit. PA1 f.sub.12W : the composite focal length of the 1-st and 2-nd lens units at the wide end; and PA1 f.sub.3 : the focal length of the 3-rd lens unit. PA1 f.sub.3 : the focal length of the 3-rd lens unit; and PA1 f.sub.4 : the focal length of the 4-th lens unit. PA1 .gamma.: the zoom ratio (=the focal length at the tele end divided by the focal length at the wide end); PA1 X.sub.2 : the amount of movement of the 2-nd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; and PA1 X.sub.3 : the amount of movement of the 3-rd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end. PA1 .gamma.: the zoom ratio (=the focal length at the tele end divided by the focal length at the wide end); PA1 X.sub.3 : the amount of movement of the 3-rd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; and PA1 X.sub.4 : the amount of movement of the 4-th lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end. PA1 f.sub.1 : the focal length of the 1-st lens unit; PA1 f.sub.12W : the composite focal length of the 1-st and 2-nd lens units at the wide end; PA1 f.sub.3 : the focal length of the 3-rd lens unit; PA1 f.sub.4 : the focal length of the 4-th lens unit; PA1 .gamma.: the zoom ratio (=the focal length at the tele end divided by the focal length at the wide end) PA1 X.sub.2 : the amount of movement of the 2-nd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; PA1 X.sub.3 : the amount of movement of the 3-rd lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end; and PA1 X.sub.4 : the amount of movement of the 4-th lens unit when zooming is effected from the wide end (taken here as a standard position) to the tele end. PA1 .beta..sub.2T : the lateral magnification of the 2-nd lens unit at the tele end. PA1 .beta..sub.5 : the lateral magnification of the 5-th lens unit. PA1 f: the focal length of the entire system; and PA1 F.sub.NO : the F-number of the entire system.